1. Field of the Invention
The present invention relates to a magnetic sensor unit for use in for example a banknote identifying apparatus for identifying, the denomination of a banknote by using magnetically biased magnetoelectric transducers.
2. Description of the Prior Art
A magnetic sensor using magnetoelectric transducers has been used to read characters and symbols which are printed on banknotes and bonds with magnetic inks and the like. Since magnetoelectric transducers have low impedances, no inductance component, and high sensitivity, they have been used in place of conventional magnetic heads.
FIG. 1 shows a circuit arrangement of a magnetic sensor of this type. Reference numeral 1 denotes a permanent magnet. Reference symbols MR1 and MR2 denote two series-connected magnetoelectric transducers which are magnetically biased by the permanent magnet 1. A voltage Vin is applied to one end of the pair of magnetoelectric transducers MR1 and MR2, and the other end of the pair is grounded. The upper portions of the two magnetoelectric transducers MR1 and MR2 are covered with a metal cover 2 with a small gap.
Assume that a very small magnetic element M comes from the left of the metal cover 2 and moves over the metal cover 2 in a direction indicated by an arrow, approaches the magnetoelectric transducers MR1 and MR2 in this order, and moves to the right. In this case, since the strengths of magnetic fields acting on the respective magnetoelectric transducers are changed, an output voltage Vout, which is changed as shown in FIG. 2, is generated from an output terminal 3 connected to the node between the magnetoelectric transducers MR1 and MR2. The presence of the magnetic element M can be detected by measuring this voltage change.
For example, a magnetic sensor unit disclosed in Japanese Laid-Open Utility Model No. 61-49466 is known as a magnetic sensor unit utilizing such a principle. FIG. 3 shows such a magnetic sensor unit.
A magnetic sensor unit 100 comprises a rectangularly parallelepiped sensor case 4 made of a nonmagnetic material such as a plastic or aluminum material, and detecting units 5 which are respectively housed in a plurality (three in FIG. 3) of unit fitting holes 4a formed in the sensor case 4 in its longitudinal direction. Both the longitudinal side portions of the upper surface of the sensor case 4 constitute inclined surfaces 4b on which the end faces of a banknote guide plate (not shown) are positioned. A banknote S (indicated by an alternate long and short dashed line) is guided by the banknote guide plate and passed over the upper surface side of the sensor case 4 in a direction indicated by an arrow D.
As is apparent from FIGS. 4 and 5, each detecting unit 5 comprises: a stem 10 made of a nonmagnetic material such as a plastic material; a magnet 11 which has upper and lower surfaces respectively magnetized to the S and N poles and is housed in a magnet housing recess 10a which is formed in the stem 10 so as to extend from its bottom surface to its upper surface side and to have a substantially rectangular or circular section; a magnetic sensor 12 which is housed in a sensor housing recess 10b, which is formed in the banknote passage surface side (upper surface side) of the stem 10 so as to have a substantially rectangular section, and is magnetically biased by the magnet 11; a plurality of terminal pins 13 for connecting the magnetic sensor 12 to an external part; and an outer cover 14 made of wear resistant non-magnetic metal such as tungsten and phosphor bronze which is fitted on the unit from the upper side of the stem 10 and covers the upper surface and side surfaces of the stem 10 while the magnetic sensor 12 is mounted in the unit. A molding material 15 is charged into the unit from the lower surface side of the outer cover 14 in order to fix the stem 10 in the outer cover 14. The magnetic sensor 12 comprises a ceramic substrate 12a housed in the sensor housing recess 10b, and a pair of magnetoresistive elements 12b fixed on the substrate 12a and magnetically biased by the magnet 11. The magnetoresistive elements 12b are connected to the terminal pins 13, respectively.
When the detecting unit 5 is to be housed in the unit fitting hole 4a of the sensor case 4, the magnetoresistive elements 12b are regularly arranged at predetermined positions in the X direction perpendicular to a detection line B--B and the Y direction parallel thereto.
In the magnetic sensor unit 100 assembled in this manner, when a banknote comes from a direction indicated by an arrow D in FIG. 5, the magnetic sensors 12 of all the detecting units 5 detect the magnetic ink of the banknote and output detection signals.
In such a conventional magnetic sensor unit, however, when the sensor unit 4 is molded, manufacturing dimensional errors of the unit fitting holes 4a are caused upon molding, and the stem 10 and the outer cover 14 exhibit variations in dimension for each detecting unit 5. Therefore, when the detecting units 5 are respectively fitted in the unit fitting holes 4a of the sensor case 4, gaps X1, X2, and X3 appear in the X direction perpendicular to the detection line B - B, and at the same time, gaps Y1, Y2, and Y3 appear in the Y direction parallel to the detection line B--B, as shown in FIG. 5. As a result, the magnetic sensor 12 of each detecting unit 5 may be deviated from the detection line B--B. The phenomenon that the magnetic sensor 12 of each detecting unit 5 is deviated from the detection line B--B also occurs when manufacturing errors are caused in terms of, e.g., the dimension of each stem 10 and the thickness of each outer cover 14.
Assume that a reference detection plane E--E of the sensor case 4 is set when a banknote S comes from the direction indicated by the arrow D in FIG. 5. In this case, a distance d between the reference detection surface E--E and the detection line B--B varies within the ranges of the gaps .DELTA.Xl, .DELTA.X2, and .DELTA.X3. In addition, positional errors of the magnetic sensors 12 are caused within the ranges of .DELTA.Y1, .DELTA.Y2, and .DELTA.Y3, respectively, in the Y direction along the detection line B--B. As a result, the detection line B--B varies for each detecting unit 5. Hence, accurate detection cannot be performed, and the insertion of the banknote must be repeated. That is, in the conventional technique, in order to perform accurate positioning of each magnetic sensor 12 without a positional error with respect to the detection line B--B, the following procedure is required. The dimensions of the unit fitting holes 4a of the sensor case 4 are matched with each other. The dimensions of the stems 10 of the respective detection units 5 are accurately matched with each other, and so are the thicknesses of the respective outer covers 14. In addition, each magnetic sensor 12 must be accurately positioned to a corresponding stem 10 to be mounted. In order to mount the magnetic sensors 12 of all the detecting units 5 without positional errors with respect to the detection line B--B, high-precision processing and mounting techniques are required.